1. Field of Invention
The present invention relates to a technical field of sludge treatment, and more particularly to an urban sludge dehydrating system and a method thereof.
2. Description of Related Arts
Urban sludge is a byproduct of city sewage treatment, which is a general designation of substances such as a small amount of sediments, particulate matters and floating matters formed during urban sewage treatment with conventional methods. The urban sludge mainly has the following characteristics:
(1) large amount and low harmlessness rate: an annual output of sludge is 28 million tons (with a water content of 80%, according to year 2011), wherein ¾ thereof is not effectively treated;
(2) high water content, and moisture which is difficult to be removed: according to conventional mechanical dehydrating method of sludge, a water content of the sludge is only able to be decreased to about 80%;
(3) harmfulness and usefulness: there are a variety of organic pollutants and heavy metals in the urban sludge, so direct emission is harmful to environments; however, with a high calorific value (7500-15000 kJ/kg for dry sludge), the urban sludge is applicable to resource utilization.
Conventionally, composting, land-filling and incinerating are common methods for sludge treatment, all of which have certain requirements on the water content of the sludge. For example, sludge landfill needs a water content of less than 60%, sludge compost needs to add a swelling agent for adjusting a water content to about 50%, and sludge incineration needs to reduce a water content to about 30%. Therefore, with the water content of about 80%, the wet sludge must be further dehydrated.
A thermal drying method is suitable for further dehydrating the sludge, which is able to reduce the water content of the sludge to less than 50% with sufficient volume reduction effect and product stability. However, the method has high technology requirements, complex management, large energy consumption, and high treatment costs. According to the sludge thermal drying method, evaporation of 1 ton water consumes 887025 kcal heat which equals to 107 m3 natural gas with a heat value of 8300 kcal. Supposing that a water content of the wet sludge is 80% and needs to be dried to 10%, then 1 ton dry sludge consumes approximately 428 m3 natural gas and 300 kW·h electricity, which means a dehydrating cost of 1 ton dry sludge is up to 1144.9 RMB (excluding staff salaries, depreciation, etc.). Therefore, even equipments are built on, production cost is unaffordable. Accordingly, an economical method is urgently needed to replace the conventional thermal drying method.
According to sludge hydrothermal treatment technology, the sludge is heated so as to hydrolyze viscous organics in the sludge at a certain temperature and pressure, for destroying a gel structure sludge, improving dehydrating performance and improving anaerobic digestion performance. The technology is also known as thermal conditioning. The hydrothermal treatment technology is divided into thermal hydrolysis and wet oxidation in accordance with whether oxidant is added. The thermal hydrolysis needs no oxidant, while the wet oxidation needs to add oxidant into a reactor.
The sludge thermal hydrolysis technology further efficiently and economically reduces the water content of the sludge, which is based on a cell disruption principle, and is able to dewater and dry the sludge with low power consumption and high efficiency. However, conventional thermal hydrolysis systems are easy to be blocked, and volume reduction effect thereof is ineffective.